It's not one workflow, and there is no scenario in which all 400 tabs are active at once.
Bookmarks take an extra step to save to, an extra step to load from, and do not stay in sync as I browse. Synchronizing a bookmarks folder with 10-20 tab changes would take significant human overhead.
Bookmarks are also slower to review than tabs, if you need to see anything besides the name/url/icon. You would need to load the bookmark into a tab before viewing it. Tabs are already in a tab, though not necessarily loaded.
Bookmarks take an extra step to save to, an extra step to load from, and do not stay in sync as I browse. Synchronizing a bookmarks folder with 10-20 tab changes would take significant human overhead.
Bookmarks are also slower to review than tabs, if you need to see anything besides the name/url/icon. You would need to load the bookmark into a tab before viewing it. Tabs are already in a tab, though not necessarily loaded.